Known relevant prior art is summarized below with reference to microlithography apparatus that employ an electron beam as a representative charged particle beam for performing projection transfer of a pattern defined by a reticle.
Projection-exposure microlithography using an electron beam exhibits high accuracy but low "throughput" (i.e., number of wafers that can be processed per unit time). Various technical approaches have been considered for solving this problem. One current approach is termed "cell projection" which is predominantly used whenever the pattern to be transferred comprises a relatively large area in which a particular small portion of the pattern is repeated many times (such as in a pattern for a memory chip comprising a large number of identical memory cells wherein each memory cell represents the repeated small portion). The highly repeated portion of the pattern is represented by a cell (approximately (5 .mu.m).sup.2 on the wafer) that is exposed multiple times on different respective regions of the wafer. Unfortunately, however, portions of the pattern that are not in the highly repeated region must be transferred using another technique such variable-shaped-beam writing. The need to change exposure methods to achieve exposure of the entire pattern and the inherent slowness of beam writing result in cell projection having, usually, an unacceptably low throughput.
Another conventional approach that has been considered offers much greater throughput. In this technique, the reticle defines the entire pattern to be projected onto a die region on the wafer in one exposure or "shot" of the electron beam. The transferred image is typically "demagnified" or "reduced" by which is meant that the image formed on the wafer is smaller than the corresponding pattern on the reticle by a "demagnification factor." The demagnification factor is usually an integral ratio such as 1/4 or 1/5. Unfortunately, exposing an entire die pattern in one shot cannot be performed with sufficient accuracy, mainly because the charged-particle-beam optics must be very large to accommodate a field the size of an entire die. Such large optics exhibit excessive aberrations, especially along the edges of the exposure field. Furthermore, producing a reticle for such exposure is extremely difficult.
Yet another conventional approach involves dividing the reticle pattern into multiple "exposure units" (pattern portions such as "subfields"). Such a reticle is termed a "divided" or "segmented" reticle. The exposure units are exposed individually in an ordered manner using a projection-optical system having a relatively large optical field (but still much smaller than would be required for single-shot exposure of an entire die pattern). Such a technique is termed "divided-pattern" projection-transfer. As each exposure unit is projected onto the wafer, aberrations exhibited by the projection-optical system (e.g., distortion and shifts in the focal point) are corrected. Hence, divided-pattern projection-transfer offers prospects of high resolution and transfer accuracy over the entire pattern as transferred to the wafer, compared to single-shot transfer of an entire die pattern.
On a "divided" reticle, the pattern is divided into multiple exposure units typically having a uniformly fixed area (e.g., sized to produce corresponding exposure regions measuring (250 .mu.m).sup.2 on the wafer). Each exposure unit on the reticle is typically separated from adjacent exposure units by "struts" (reinforcing crosspieces) flanked by "skirts" (non-patterned regions). The struts provide structural rigidity to the reticle.
Normally, most if not all the exposure units of a pattern to be exposed are different from each other and have different arrangements or distributions of pattern features and/or different "feature densities." Such differences from one exposure unit to another, and from one subregion to another subregion within the same exposure unit, can cause a corresponding substantial change in beam current passing through the exposure unit or subregion of an exposure unit to the wafer. I.e., even with a constant beam current illuminating the exposure units on the reticle, the beam current actually passing through an exposure unit, or through different subregions of a single exposure unit, can vary substantially from one exposure unit to the next or from one subregion of an exposure unit to another. Accompanying such changes in beam current usually are changes in Coulomb interactions (Coulomb effects) among the electrons in the beam propagating from the reticle to the wafer. Generally, the higher the localized beam current, the greater the corresponding localized Coulomb effect. A change in the magnitude of the Coulomb effect is manifest as a corresponding change in focus of the image on the wafer, such as from one exposure unit to the next or from one region to the next within a single exposure unit.
Focal shifts, especially those occurring from one exposure unit to the next, can be corrected to some extent by making compensating adjustments of current or voltage applied to a lens through which the beam passes. However, Coulomb effects arising from variations in feature density or feature distribution within individual exposure units are difficult to correct in this manner. I.e., an ideal focus setting for a particular exposure unit may not achieve ideal focus for all the features defined in the exposure unit because the ideal focus for one feature may not be ideal for another feature in the exposure unit. Hence, an ideal focus condition normally is achieved only for a small region within each exposure unit, and conventional efforts to improve this situation for other regions of each exposure unit using a focusing lens typically results in a deterioration of overall exposure accuracy of the exposure unit.
Conventional methods directed at reducing the magnitude of focus shift due to Coulomb effects include reducing the beam current used to illuminate each exposure unit of the reticle and reducing the size of each exposure unit. Unfortunately, however, both methods reduce throughput, making them impractical for industrial use.